1. Field of the Invention
The present invention relates to a cross dipole antenna to be applied to an RFID (radio frequency identification) system for communicating information between a reader/writer and a tag using a high-frequency radio signal, and a tag using the same.
2. Description of the Related Art
In the RFID (radio frequency identification) system, a carrier wave signal of approximately 1 W is transmitted from a reader/writer by use of a radio frequency signal in the UHF band (860-960 MHz). The tag side receives the above carrier wave signal, and modulates the above carrier wave signal with tag information. The modulated signal is returned to the reader/writer side as a response signal, and the information in the tag is read by the reader/writer. Application fields of such the RFID system are spreading now.
The communication distance in the above system ranges approximately 3-5 meters, although depending on an antenna gain provided in the tag, an operating voltage of an LSI chip for signal processing, and also a surrounding environment. The tag is constituted of an antenna pattern of conductive material, being formed on a base body such as a sheet and a film having a thickness of the order of 0.1 mm, and an LSI chip (approximately 1 mm square with a thickness of 0.2 mm or so) connected to an antenna feed point.
As shown in FIG. 1, an LSI chip 2 can be expressed equivalently by a parallel connection of a resistance Rc (500Ω as an example) and a capacitance Cc (1.4 pF as an example). Meanwhile, a tag antenna 1 can be expressed equivalently by a parallel connection of a radiation resistance Ra (400Ω as an example), and an inductance La (20 nH as an example).
By connecting the above LSI chip 2 and tag antenna 1 in parallel, the capacitance resonates with the inductance, resulting in matching at a desired resonant frequency f0, as can be understood from formula 1 shown below. Hence, it follows that the receiving power in antenna 1 is sufficiently supplied to the chip 2 side.f0=1/2π√{square root over (LC)}  (formula 1)
Now, in FIG. 2A, a basic configuration of the dipole antenna is shown. Since an overall antenna length is set to λ/2 of a use frequency, in case of 953 MHz, the overall antenna length is a λ/2 wavelength, which equals approximately 145 mm. LSI chip 2 is connected at a feed point of each antenna element having a length of λ/4.
By setting the overall antenna length to λ/2, the antenna resonates at 953 MHz. However, there is no La component shown in FIG. 1. Therefore, as shown in FIG. 2B, by setting the length to 180 mm or of that order, which is longer than λ/2, antenna 1 comes to have an La component, and thus it becomes possible to make antenna 1 resonate with LSI chip 2.
As such a dipole antenna formed on an RFID tag, it has been known to make the antenna length longer than λ/2, for example, in the publication, “Antennas and Propagation International symposium”, 2005 IEEE Volume 2B, 3-8 pages 353-356, July 2005.
Further, since the polarization of the above tag antenna 1 is linear, usually, circular polarization is used for a non-illustrated antenna on the reader/writer (R/W) side, so as to enable transmission and reception at a substantially identical distance even when the tag is rotated in a plane parallel to a paper plane. Namely, assuming to use a linear polarization antenna on the R/W side, when the polarization direction thereof coincides with a tag polarization direction, the communication distance becomes longer by approximately 1.4 times, as compared to the a case of using circular polarization. In contrast, the communication distance becomes drastically decreased when the polarization direction of the R/W antenna is placed perpendicular to the tag polarization direction.
Now, an ordinary chip has two terminals as shown in FIG. 1. Furthermore, by connecting the two systems in parallel, a chip having four terminals is also put into the commercial market today, as shown in FIG. 3. In many such cases, the grounding sides (GND) are mutually connected in a direct current (DC) manner. Therefore, for example, by configuring the antenna with two systems of cross structure using dipole antennas shown in FIG. 1, it becomes possible to maintain the communication distance long even when a linear polarization antenna is used on the R/W side. However, the vertical and horizontal lengths come to 180 mm, respectively. Such an antenna is too big for a tag antenna of practical use.